The need to sense electric field vectors, or potentials with respect to a ground plane, arises in a variety of circumstances, and has been met in the prior art by the use of antennas. One well known use of antennas is for the sensing of radio waves. Such radio waves may be intelligence-bearing modulated signals produced by a transmitter, or the result of atmospheric disturbances, such as lightning, as disclosed, for example, in U.S. Pat. No. 4,023,408. Although electric field sensing can be associated with a variety of structures and vessels, e.g., buildings and ships, the provision of electric field sensors for aircraft has been beset with problems unique to the aircraft environment.
The desire to increase the dimensions of the sensor, for good performance, e.g., signal to noise ratio, is at odds with the desire, in the aircraft environment to minimize the extension of the antenna beyond the airframe because such extensions reduce the aerodynamic performance of the aircraft.
Accordingly, the provision of a low profile antenna with good electrical properties, i.e., signal to noise ratio, is an advantage in the aircraft environment; it is also an advantage in non-aircraft environments for the reasons of economy, reduction of complexity and aesthetics.
A transmitter of electromagnetic radiation (either intelligence-bearing or natural) will produce a time varying electric field at a distant location. The electric field vector is sensed by detecting a potential with respect to the local ground plane which is induced in the sensor because of the field. To sense the presence of the electric field, the potential at a point in space is measured with respect to the local ground plane. This potential can be measured, for example, by the use of an antenna which is simply a metallic structure in which a potential is induced by the electric field. Since the antenna is a real (as opposed to an ideal) structure, it extends over an infinite number of points in space, each of different distance from the local ground plane. The potential actually induced in the antenna then is the integral of the potential induced at an infinite number of points P on the antenna, each a different distance from the local ground plane. Since the length of the antenna is assumed to be a small portion of a wavelength, propagation time effects can be neglected. Accordingly, we can assign an effective height H.sub.e to the antenna such that the potential induced in the antenna is the same as the potential that would be induced in the antenna if all the material in the antenna were concentrated at a distance H.sub.e from the local ground plane, that is, the potential e.sub.a is H.sub.e .sigma., where .sigma. is the electric field vector.
Because of the combined presence of two conductors, i.e., the local ground plane and the antenna, the combination will also exhibit electrical capacitance. An equivalent circuit for the antenna arrangement comprises a voltage generator (of magnitude proportional to the product of the electric field and the effective height of the antenna) in series with the capacitance of the antenna with respect to the local ground plane. Whip antennas normally used on aircraft have effective heights ranging between 0.1 and 0.25 meters, and antenna capacitance varying in a range between 10 and 50 pf. These parameters are a compromise between the desire to achieve larger effective heights, for improved signal to noise ratio, and the desire to reduce the height of the antenna to avoid disturbing the aerodynamic performance of the aircraft.
The prior art also evidences attempts to eliminate the whip, and instead use an antenna which is generally planar in shape, with a major dimension extending generally parallel to the local ground plane, i.e., a plate. Such a sensor is illustrated in FIG. 2. In a plate type sensor, which is usually oriented generally parallel to the local ground plane, the effective height of the antenna lies somewhere between the extreme edges of the plate and the local ground plane. Likewise, the antenna capacitance, then, is the capacitance between the plate and the ground plane.
With this arrangement, the effects of the previous compromise are highlighted. That is, improved aerodynamic performance can be achieved by reducing the effective height of the antenna which, in the case of the plate sensor, is approximately the actual height. However, this has a strong impact on the potential induced into the antenna which may degrade the signal to noise ratio. Furthermore, in order to prevent the capacitance of any connecting cable from further attenuating the induced potential, the prior art used a voltage amplifier co-located with the flat plate sensor, and such a voltage amplifier is also shown in FIG. 2. The resulting compromise has resulted in commercial products with antenna effective heights (H.sub.e) at least greater than 5 cm.
For comparison purposes, FIG. 3 plots noise level and voltage amplifier output voltage as a function of effective antenna height. Reviewing these two curves, it will be apparent that above some low threshold, the noise level increases in proportion to effective antenna height and voltage amplifier output also increases linearly with effective height, although the output voltage of the voltage amplifier increases with increasing effective antenna height at a faster rate than noise. At the chosen effective antenna height of about 5 cm. (i.e., point D in FIG. 3), signal to noise ratio (for about a 100 kHz noise bandwidth) is about 1.5. Better S/N is easily achieved by increasing antenna effective height.
It is therefore, one object of the present invention to provide a low profile electric field sensor which provides usable output signals, and at the same time, has an effective antenna height which is less than devices available today. It is another object of the present invention to provide low profile electric field sensor which minimizes aerodynamic disturbance, without penalty to electrical properties of the sensor.